Union and student groups said 2.6 million people took to the streets across France today to protest against a new labour law making it easier to fire young workers.

A union official said the protest against the first job contract was one of the biggest since France's fifth republic was founded in 1958. French police, however, said the turnout was much lower at 900,000.

Skirmishes broke out in Paris between riot police and hooded youths at the end of a rally that drew tens of thousands of mostly peaceful students and employees.

Police used aggressive tactics to prevent any repetition of the violence that has marred previous demonstrations against the law.

Armed with orders to make as many arrests as possible, officers made forays into the crowd to round up troublemakers. They fired teargas at youths who threw traffic cones and tagged them with guns firing indelible ink.

Big crowds of demonstrators also joined rallies in Marseille, Bordeaux, Grenoble and Lyon, as well as in other cities.

"We have to defend the rights that were won by our ancestors and which the current government is trying to take away," Maxime Ourly, a literature student, told the Associated Press.

Commuter trains, buses and metro services were disrupted as unions went on a one-day national strike to oppose the "first job contract" law.

Postal staff, teachers and media employees also joined the action.

It was the first time that unions had ordered walkouts in solidarity with students who have spearheaded the recent wave of protests.

The dispute could have a major impact on next year's presidential elections, with the conservative prime minister, Dominique de Villepin - whose government introduced the controversial law - a potential candidate.

Mass street protests over pension reforms in 1995 were widely believed to have lost the conservatives the legislative elections held two years later.

Unions and students want Mr De Villepin to cancel the first job contract law, which is due to take effect next month.

They say the legislation will create a generation of throwaway workers because it will allow companies to dismiss workers under 26 without cause in their first two years of employment.

Mr De Villepin insists the measure will help reduce France's high levels of youth unemployment by giving bosses more flexibility.

French workers benefit from strong job protection, but some believe that discourages employers from hiring less experienced staff who have not yet shown they can do a job.

But a crack in the government opened when interior minister Nicolas Sarkozy, also a likely presidential candidate, suggested that the contract be suspended to allow talks with unions - a clear break from Mr De Villepin.

Today's strike brought around half the commuter trains and 30% of metro services in the Paris area to a standstill. Public transport in 76 cities across the country was affected.

"It's really annoying," secretary Monique Paquet told the Associated Press as she waited for a bus in the Opera area of the French capital. "These strikes really slow me down."

Many French people have grown accustomed to walkouts by transport workers and adapted by taking time off or rearranging commutes to avoid peak hours.

Jean-Paul Boulet, a spokesman for the national train operator SNCF, said packed train stations were "a thing of the past", adding: "People get organised or will stay at home."

One flight in three at airports nationwide was disrupted, and the state-run radio station France-Info, a major source of daily news, broadcast only music.

People flying to France from Britain faced cancellations, with the budget airline Ryanair saying it had axed more than 70 flights from Stansted, Luton and Liverpool.

In an attempt to break the standoff, Mr De Villepin has offered to meet unions and student groups tomorrow. Most major unions want the first job contract measure withdrawn before any talks can begin.